This invention relates to measurement of fluid flow characteristics, and more particularly, to a method and apparatus for measuring the density and mass flow rate of a flowing fluid. It is well known that a bladed rotor will rotate at a speed which is directly related to the velocity of the fluid flowing past the rotor. The proportionality constant between the rotation rate of a particular rotor and the volume flow rate of the fluid is sometimes called the rotor's K-factor.
Although the rotation rate of a single rotor can be a measure of the volumetric flow rate of a fluid past the rotor, a measurement of mass flow rate or fluid density may be desired. To measure mass flow rate, an apparatus has been used whereby a bladed rotor is forced to rotate at a fixed rate, the torque required to make it do so being proportional to the rate at which fluid mass is passing the forced rotor. A different apparatus has also been developed, described in detail in U.S. Pat. Nos. 2,943,487 and 3,144,769. In general, the apparatus disclosed in these patents consists of two bladed rotors, one with a K-factor greater than the other, the two rotors being coupled together via a helical spring mechanism along the central axis. If the two rotors were not coupled, the one with the higher K-factor would turn faster than the one with the lower K-factor. The spring coupling, however, makes the high K-factor rotor induce a restoring torque on the low K-factor rotor, thereby pulling the low K-factor rotor around with it. The result is that both rotors turn at the same speed, but the rotor with the higher K-factor will lead the other rotor by a certain angular displacement, .theta.. It has been found that this angle is directly proportional to the momentum of the fluid passing the two rotor system. Being directly proportional to momentum, it is also directly proportional to mass flow rate. The larger the fluid momentum, the larger the phase angle, and the larger the induced torque on the low K-factor rotor.
A system which utilizes this relationship to measure mass flow produces an electrical impulse as each rotor passes through a fixed reference plane extending radially from the common axis of the rotors. These impulses are produced when a permanent magnet, such as a magnetized vane of a rotor, passes a pickup coil held in a fixed position adjacent to the rotor but external to the conduit in which the fluid is flowing. The angular placement of these pickup coils define the orientation of the fixed reference plane mentioned above. Since the higher K-factor rotor turns slightly ahead in phase from the rotor with the lower K-factor, the impulse from the higher K-factor rotor is produced earlier in time than the impulse from the lower K-factor rotor. This prior art teaches the measurement of mass flow rate by measuring solely the time period between the production of these two impulses.
Two problems have been found with this system. First, the time period between the two impulses above described is not a direct measure of the mass flow rate of the fluid. Second, coupling the rotors via a centrally located helical spring mechanism is mechanically complex and clumsy. This makes manufacturing difficult and expensive, and can lead to low reliability.